2003-09-05 04:00:00 PDT Monterey -- Two scientists quoted no less a poet than Robert Frost to underscore an extraordinarily gloomy long-term forecast for the future of both Earth and Mars, in a presentation to astronomers here Wednesday.

"Some say the world will end in fire, Some say in ice," Frost wrote in 1920.

In a mere 7.5 billion years from now, both will inevitably come true, according to Jeffrey S. Kargel, an astronomer-geologist with U.S. Geological Survey, and Bruce Fegley Jr., a professor of planetary chemistry at Washington University in St. Louis.

The bright and pleasant sun of today, they said, will grow into a "red giant" so hot and huge it will "almost but not quite engulf the Earth" and create a catastrophe that's unimaginable.

The sun's energy will multiply by 2,800 times. The solar heat will melt the entire crust and inner mantle of both Earth and Mars, creating vast oceans of molten rock hundreds of miles deep.

All life on Earth, of course, will long since have become extinct -- on Mars, too, if there is any -- and temperatures will rise: first by 3,000 degrees Fahrenheit, which is hotter than any known volcano, and ultimately, some 68 million years later, to at least 4,000 degrees.

"Earth will glow like a star itself," said Kargel, while the Martian temperature will merely reach the heat of molten basalt.

"Strange continents of sodium, potassium, aluminum and calcium will float on the molten ocean, and glaciers of silicon, iron and magnesium will flow down from the continents into the molten sea," Kargel said.

If that does happen, they predicted, the side of Earth in shadow would have temperatures more than 400 degrees below zero Fahrenheit, all the while whipped by tempestuously hot winds sweeping down from the sunlit side.

"It will be a hellacious place," said Kargel. Kargel is a space scientist with the astrogeology program of the U. S. Geological Survey in Flagstaff, Ariz. While Kargel spoke at the meeting in Monterey, Fegley spoke to The Chronicle by telephone from his office at Washington University.

Fegley's main role, he said, was to provide Kargel the math and chemistry software for their predictions. Much of their conclusions were based on data from satellites and space probes that have examined the chemistry of comets and meteors -- plus observations of the violent volcanic activity observed by the Galileo spacecraft on Io, a major satellite of the planet Jupiter.

"Kargel's conclusions are extremely innovative," Fegley said, "and they need to be followed up. I'm very confident in the calculations, and I think he has a great idea."